Until recently, virtually all the oil produced in the world was recovered by primary methods, which relied on natural pressures to force the oil from a petroleum reservoir. Natural pressures within a petroleum reservoir cause oil to flow through the porous rock into wells and, if the pressures are strong enough, up to the surface. However, if natural pressures are initially low or diminish with production, pumps or other means are used to lift the oil. Recovery of oil using natural pressures is called primary recovery, even when the oil has to be lifted to the surface by mechanical means.
As new fields have become increasingly difficult and more costly to find and oil prices have risen, the stimulus to increase recovery from known fields has steadily become stronger. Enhanced oil recovery research has been conducted for many years and commercial application of these procedures is becoming more and more feasible. Enhanced oil recovery processes begin with four basic tools: chemicals, water, gases and heat. Of importance are the in-situ combustion method, which uses heat as a basic tool, and miscible recovery, using carbon dioxide as a basic tool.
The in-situ combustion method produces heat energy by burning some of the oil within the reservoir rock itself. Air is injected into the reservoir and a heater is lowered into the well to ignite the oil. Ignition of the air/crude oil mixture can also be accomplished by injecting heated air or by introducing a chemical into the oil-bearing reservoir rock. The amount of oil burned and the amount of heat created during in-situ combustion can be controlled to some extent by varying the quantity of air injected into the reservoir.
The physics and chemistry of in-situ combustion are extremely complex. Basically, the combustion heat vaporizes the lighter fractions of crude oil and drives them ahead of a slowly moving combustion front created as some of the heavier unvaporized hydrocarbons are burned. Simultaneously, the heat vaporizes the water in the combustion zone. The resulting combination of gas, steam and hot water aided by the thinning of the oil due to the heat and the distillation of the light fractions driven off from the oil in the heated region moves the oil from injection to production wells.
Carbon dioxide miscible recovery may be used, although carbon dioxide may not be initially miscible with crude oil. But, when the carbon dioxide is forced into an oil reservoir, some of the smaller, lighter hydrocarbon molecules in the contacted crude will vaporize and mix with the carbon dioxide, forming a wall of enriched gas consisting of carbon dioxide and light hydrocarbons. If the temperature and pressure of the reservoir are suitable, this wall of enriched gas will mix with more of the crude forming a bank of miscible solvents capable of efficiently displacing large volumes of crude oil ahead of it. Additional carbon dioxide is injected to move the solvent back toward the producing wells.
Traditionally, carbon dioxide is found in underground deposits and can be produced through wells similar to gas wells. Normally, however, the carbon dioxide must be transported to the oil reservoir, which can add significantly to the cost of this enhanced oil recovery process.
Natural gas and air have also been used in the miscible gas injection processes to aid in the secondary recovery of oil from known reservoirs. In addition, chemicals, such as alkalis, polymers and surfactants have been used in conjunction with water flooding to aid in recovery of crude.
A problem with the methods of enhanced oil recovery presently known is that at a given reservoir, only one method of enhanced oil recovery will be used at a time.